More Info: The freezing point of water is affected by the purity of the water and to a lesser degree atmospheric pressure.
Will Additives In Water Change Its Freezing Point?
Additives in water can lower its freezing point in a phenomenon known as freezing point depression. When you add a solute (for example, salt) to a solvent (for example, water), the solvent’s freezing point decreases. This occurs because the newly created solution will undergo physical and chemical changes. In chemistry, these changes are referred to as colligative properties. The new freezing point will depend on the number of new particles added in relation to the number of solvent molecules present in the new solution. 
For example, a 10% salt solution will lower the freezing point of water to 20 degrees F (-6C) 
How Does Atmospheric Pressure Affect the Freezing Point?
Atmospheric pressure is the pressure exerted by the atmosphere at the Earth’s surface. At sea level, it has a mean value of one atmosphere, reducing with increasing altitudes. 
Atmospheric pressure plays a small role in the temperature at which water will freeze. At higher elevations as the atmospheric pressure decreases, the freezing point of water will rise slightly. At lower elevations, the pressure increases resulting in a lower freezing point. 
According to the US Department of Energy’s Office of Science, the effect is minimal as the freezing point of water only rises one-hundredth of a degree per atmosphere of decrease in pressure. 
At What Temperature Does Ocean Water Freeze?
Ocean water freezes at 28.4°F (-2°C).
Ocean water freezes at a lower temperature than fresh water due to its salt content, which interferes with the water’s ability to form ice. A look at how ice is formed will help to clarify why the addition of salt, or any solvent, requires a lower temperature to freeze. 
When water is exposed to low temperatures, the molecules in motion begin to slow down making it easier for the molecules to share electrons. As more and more pair up, they begin to form hexagonal structures that interlock forming ice. With the addition of salt to this process, the salt particles become intertwined with the water molecules, interfering with their ability to share electrons and interlock. When the ice does ultimately freeze at a lower temperature, it expels the salt. 
Why Don’t All Oceans Freeze in the Winter?
First, it’s important to note that under the right climatic conditions some oceans do freeze such as those in the Arctic and Antarctica. In the Northern Hemisphere, oceans form ice as far south as Bohai Bay, China. 
If oceans do freeze, as is demonstrated in the Arctic and Antarctic, why don’t all oceans freeze like lakes? The actual science behind the answer involves a variety of complex processes far beyond the scope of this article. At its most basic level, sheer volume and ocean currents can be partially identified as causes.
It takes an extended period of frozen temperatures to cool a heated water mass to the freezing point. A puddle will freeze before a lake, which will freeze before an ocean due to its sheer mass.
New England, for example, has cold weather, but not cold enough for an extended period to form an ice layer on its oceans. The Arctic, on the other hand, has long winters with an average temperature of -40°F, capable of producing ice. 
Ocean currents also play a role in keeping ocean water from freezing. The movement, as well as the warmer waters introduced to the colder waters, inhibit ice from forming.